It has been known for many years that plants can be grown without natural soil by substituting a direct source of nutrients. The term "hydroponics" has been coined to describe a variety of soilless culture systems. The basic concept in hydroponics is to feed chemical nutrients, i.e. nitrogen, potassium, calcium, etc., directly to the plant's roots through the vehicle of an aqueous solution.
Hydroponic techniques have been used in the laboratory for decades to achieve precise control of growth variables for scientific study. In recent years, several products have been developed to enable both private and commercial growers to utilize hydroponic principles. When hydroponic equipment and supplies are low enough in cost, carefully controlled hydroponic culture has significant economic advantages over conventional soil culture: plant nutrition is easily optimized; soil-borne diseases are eliminated; there are no weeds to deal with; and the yield and quality is more predictable and generally of better quality. Overall, this means significantly greater yield per square foot of growing area.
Hydroponic culture is usually coupled with greenhouse facilities in an effort to achieve maximum control of growth factors (temperature, light, humidity, nutrients, and water.) The art may be divided into two basic categories, open systems and closed systems. In open systems the nutrient solution fed to the plants is not retrieved but, rather, leaches and drains away. Fresh nuturents are continually supplied and, with the use of "nutrient concentrate injectors" (mechanisms for supplying nutrients to a water irrigation system on a continual basis), can be regulated with great precision to the plant's growth needs. Closed systems retain the nutrient solution for reuse. The solution is pumped from a reservoir to the plants where it wets the roots through one mechanism or another, and is ultimately returned to the reservoir. The "batch" solution is generally used for a one to four week period, and then discarded and replaced with a fresh batch. In both open and closed systems the nutrient solution is usually applied intermittently. This is done to insure that the roots receive an adequate amount of oxygen.
Open systems have, to date, been the most commonly used hydroponic method. They are admittedly very inefficient in their nutrient usage, in that a great deal of good nutrients are lost. But they are relatively simple mechanically, and have been very practical.
Closed systems offer potentially greater efficiency of nutrient usage and lower cost per yield. Achieving this efficiency, and consequent profitablity, in practice depends largely on the cost and practical design of the supporting equipment.
A wide variety of closed system structures and techniques have already been developed. In some, inert media are used for plant support. Such media incude gravel, sand, sawdust, vermiculite, and even synthetic plastic materials. These media have no nutrient value themselves, but merely serve as a "housing" for the plant roots and a temporary "sponge" to keep the plant roots in contact with the nutrient solution as well as with oxygen. A medium is not essential, however. One of the most successful techniques, called Nutrient Film Technique (NFT), supports plants single file along a trough and trickles nutrient solution along past the roots. Actually, the roots usually tend to form a rather dense mass that retains solution nicely while permitting oxygenation. Another technique, often called "aeroponics," suspends the plant roots freely in air, and mists them with a sprayed nutrient solution.
While many of these techniques have been successfully used by commercial and private growers, there are several disadvantages and inefficiencies which have not been overcome by the prior art. A major shortcoming of most prior art systems is the high proportion of manual labor costs. Even though the hydroponic system itself eliminates several manual operations necessary in conventional culture, the set-up, planting, plant maintenance, and harvesting are stll very labor intensive, and hence costly. Automation of these functions could result in significantly increased profitablity, but presently available techniques cannot readily be adapted to automation.
Techniques that utilize a medium which may be considered essential for root crops suffer from a number of problems. Adequate drainage can be difficult to achieve. Inefficient drainage can cause oxygen starvation and promote rotting. Initial cost of covering a large area with special media can amount to a substantial sum. That is especially evident when considering that the media must either be replaced for each crop or cleaned and sterilized. If the media is reused, sterilization adds to the cost. Build up of fine root hairs in reused media will eventially require its replacement. Some media have natural contaminants which must be dealt with before or during use. Others require physical attention, such as the sharp edges on gravel that can cut or injure plants. A generally ignored drawback to most media is their weight, which, if the medium is kept partly fluid, tends to bouy plant roots up to the surface.
In mediumless systems, and even in those using a medium, the thick mat of maturing roots can often impede proper drainage. This problem is commonly encountered in NFT. The roots can also grow into, and clog mechanical drainage ports and pathways to the detriment of proper drainage.
Prior to the present invention, an economical means of applying equal nutrient flow to all plants in a system remained an unsolved problem of the prior art. One of the closest approaches to solving this problem is trickle irrigation wherein individual feeder tubes run to each plant. However, the prior art trickle irrigation techniques often cause incomplete or nonsymmetrical nutrition because the flow is not easily focused toward the plant's center. In practice, these techniques have been clumsy to work with, since feed lines are easily knocked out of alignment by workers.
The present invention provides an improved soilless culture system which has a number of advantages over prior art systems and structures.
While the system is suited to manual operation, it is designed for automation with all of the primary functions, i.e. seedling transplant, nutrient supply, tissue sampling, foliar spraying and harvesting which can be readily accomplished with a robot tractor device. Automation permits the growth bed to be more densely arranged, permitting greater utilization of greenhouse space.
The seedling holder trays of the present invention permit faster transplant and reduce transplant shock more than any existing system. Growing seedlings in a separate chamber, more densely spaced, permits better time utilization of the main growing system.
Further, the system of the present invention can be used with or without a medium. When a medium is used, such as for a root crop, the system still benefits from advantages previously found only in mediumless systems. The initial supply cost is relatively low, since very little medium is needed, and no sterilization problems or root fiber build up need be considered because the medium can be economically discarded at the end of each growing cycle. If reuse of the medium is desired, the material flow process of this system affords a very simple cleansing procedure.
The present invention permits the use of very lightweight media, including lighter-than-water media. In conventional systems, small granular media with specific gravities much lower than 1.0 either cannot be used or must be mixed with a heavy medium because of the obvious problem of flotation. In the soilless culture system of the present invention, the medium is never completely immersed in water, eliminating problems of buoyancy and washing away of light medium.
The present invention achieves equal and individualized plant feeding at minimal cost and without the clumsiness of loose, individual feeder tubes. Individualized feeding has several advantages. It tends to minimize disease spread; it can permit maximum uniformity of plant growth; and, in this case, it totally eliminates problems of root aeration.
Further, the closed system of the present invention achieves a near theoretical minimum of water and nutrient consumption and requires a relatively small solution reservoir because the plant roots need not be flooded. They require only a trickle of liquid and the precise focusing of the solution on the plant's center minimizes the trickle required.